Wireless spread spectrum communication systems are well known. For example, in single channel spread spectrum systems, a mobile station, such as a handheld telephone, Internet appliance, laptop computer or any other suitable device may communicate with one or more base stations over a wireless air interface. Typically, a mobile station is tuned to a single channel, such as a channel that communicates voice information. Multiple mobile stations may be tuned to the same frequency but may be assigned different spreading codes, can be differentiated by time (PN offset) or other suitable differentiation criteria. A receiver, that may be in a base station or in a mobile station, typically includes a plurality of receiver channel elements, such as a plurality of RAKE receiver fingers. Each finger of a RAKE receiver is typically assigned to a different traffic channel (e.g., spreading code). A searcher typically searches a pilot channel for energy peaks to provide receiver channel element management, also known as finger management. For example, the IS95 standard known as TIA/EIA-95-B Mobile Station-Base Station Compatibility Standard for Dual-Mode Spread Spectrum Systems. utilizes pilot power that is detected by a searcher to assign different fingers to receive information during different timing windows. If pilot energy is strong enough, it is assumed that the associated traffic channel is also strong enough. Once suitably adjusted, the energy received by each of the rake fingers is suitably combined and passed to a digital demodulator, such as a Viterbi decoder. Such conventional finger management systems typically use the pilot energy per chip (Ec) per incident noise power (Io) to determine whether the searcher has detected a suitable energy peak at a particular point in time. Accordingly, the searcher provides the peak pilot energy metrics to a finger management algorithm which then provides a receiver finger control signal to adjust a receiver finger to center about a selected window in time. If the searcher determines that the pilot energies are above an allowable threshold, then the finger management system determines if a receiver finger should be assigned to the requisite path or channel. Commonly, each receiver finger is assigned to a base station. Each base station typically has one or more different assigned Walsh codes for forward link (base-to-mobile) transmissions. Also, searches keep a list of multi-paths for a channel to provide diversity reception.
A problem arises in systems with multiple simultaneous communication link capabilities such as that described in 3GPP2 C.S0002-A (CDMA 2000 System) where a mobile station may be receiving multiple channels wherein each of the channels requires a different quality of service using differing size Walsh codes. For example, a mobile unit may simultaneously require a voice channel at one rate and a data channel at a different rate to provide differing services for a user. Accordingly, a mobile station is forced to manage the radio frequency environment for each channel or service while maintaining a radio frequency link performance that is appropriate for each type of channel. Optimization of received channel energy becomes increasingly important as additional channels are required. If a user requires both a voice channel and one or more data channels at the same time, and all of the strongest received channels are voices, the user may be prevented from obtaining access to a data channel since the pilot energies for voice channels may be the highest. Where a BTS is loaded so that all data channels, for example, are already in use, even low energy data multi-path signals may be valuable for a mobile station to be able to combine with other multi-paths.
Also, unlike single channel systems, multi-channel type systems may have a varying ratio of pilot energy to traffic channel energy since they may use a closed loop transmit power control scheme. The downlink (or uplink) power can be adjusted hundreds of times per second. Accordingly, pilot energy measurements may no longer be an accurate representation of traffic channel energies due to the rapid energy changes induced by fast closed loop power control. For example, a mobile station may send a transmit power control command to a base station every 1.25 milliseconds, thereby requiring a base station, for example, to increase to decrease power of a particular channel very often. In addition, multiple channels are simultaneously active by a mobile so that multiple voice channels may be operational as well as multiple data channels. In addition, multi-channel spread spectrum systems allow the higher rate channels, such as data channels, to be turned on and off during a call. Accordingly, traffic channel energies can vary drastically on a per user basis. A mobile station may be assigned multiple codes or different length codes, so that multiple channels need to be suitably received by a plurality of receiver channel elements, such as RAKE receiver fingers. Accordingly, in multiple channel systems, both a mobile station and a base station can create transmit control information such as power control bits (PCB's) and tell each other to increase or decrease traffic channel power on a very rapid basis. Therefore, total pilot energy may not be a sufficient indicator of traffic channel energy. With more channels operating simultaneously in the rapidly changing channel power and different channel rates, determining receiver finger assignments can be quite difficult. Frequent messaging and increasing and decreasing of BTS transmit powers can necessarily increase the load on the BTS and can reduce system capacity.
In CDMA 2000 type wireless systems, time diversity over multiple receiver fingers is accompanied with a variety of different channel types served by the receiver fingers. For example, one channel type may be dedicated at a different rate for voice than for data communication. Accordingly, it is important to distinguish receiver finger performance for channels providing different qualities of service. Where there are constrained receiver resources, there may be fewer assignable fingers compared to the number of multi-path candidates. Accordingly, assigning and deassigning receiver fingers becomes an important aspect in providing improved quality of communication.
Consequently, a need exists for an improved CDMA receiver and method for managing receiver finger assignments and deassignments in systems that employ different channel types, such as channels operating at different rates.